1. Field of the Invention
The present invention relates to a radiation detection apparatus that detects radiation, such as X-rays or β-rays. More specifically, the present invention relates to a radiation detection apparatus that reduces the attenuation of radiation in a gas until radiation irradiated from a radiation source reaches an object to be measured and stabilizes the output of radiation.
Priority is claimed on Japanese Patent Application No. 2009-290614, filed Dec. 22, 2009, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
In recent years, inspection using X-rays has spread in terms of quality control in food, industrial products, and the like or security management. In many cases, it is not a soft X-ray, which is much absorbed by air, but a hard X-ray, which is less absorbed by air, that is used. However, there are also samples for which transmission characteristics can be obtained only through the use of the soft X-ray due to the properties thereof.
A soft X-ray including an extremely short ultraviolet ray can be employed in various research fields and inspection fields of food, industry, medical services, security, and the like. Further, the latent demand for the soft X-ray is very large. Since a soft X-ray is absorbed well by air or a solvent, the transmitting performance of a soft X-ray is low. Accordingly, it is difficult to measure a soft X-ray in the air or a solution. There have been many cases where it is difficult to inspect a sample having an appropriate thickness by a transmission method using a soft X-ray.
FIG. 6A is a view showing a relationship, which is disclosed at http://pfwww.kek.jp/kitajima/sx/sxme.html, between the energy of X-rays and an attenuation length λ where X-rays are absorbed by air and the energy of the X-rays is thus attenuated to 1/e. The horizontal axis represents the energy of X-rays. The vertical axis represents an attenuation length λ where X-rays having an energy of 0.02 to 30 keV are transmitted through air having a pressure of 1 atmosphere and the energy of the X-rays is attenuated to 1/e. When the energy of the X-rays is 10 keV, an attenuation length λ is about 2 m. An attenuation length λ is 0.1 m when the energy of the X-rays is 4 keV, and an attenuation length λ is 0.01 m when the energy of the X-rays is 2 keV.
Likewise, X-rays are much absorbed by a window material used in the window that partitions the atmospheric pressure and the vacuum in a radiation detection apparatus. A material, which absorbs a small amount of X-rays, has been used as the window material to a limited extent.
FIG. 6B is a view disclosed at the above-mentioned URL, and is a view showing a relationship between the energy of X-rays and the transmittance of beryllium having a thickness of 20 μm that is a typical window material. When the energy of X-rays is 5 keV or more, the transmittance of beryllium is 95% or more. When the energy of the X-rays is 1 keV or less, the transmittance of beryllium is 10% or less. Diamond or Si3N4 having a thickness of about 0.4 μm may be used as the window material.
FIG. 7A is a view showing an inspection device in the related art that uses X-rays or β-rays. A sheet-like object or sample to be inspected 3 is disposed between the beam source 1 of X-rays or β-rays and a radiation detector 2 such as a line sensor, a digital camera with a scintillator, and an ionization box. The sample 3 is installed so as to be distant from one beam source 1 by a certain distance. Radiation is radially diffused from the beam source 1 in the width direction of the sample 3, and is irradiated to the sample 3. The mixing of foreign materials, the presence or absence of components, the presence or absence of defects, and the uniformity of materials of film thickness, an ingredient, and the like are determined from the transmission characteristics of the sample 3.
If the device is further provided with a direct conversion type radiation detection element detector such as a CdTe capable of discriminating energy, it may also be possible to discriminate energy.
Transmission measurement and inspection in a relatively narrow range is used in research or the fields of food and medical services. Meanwhile, in many cases, transmission measurement in a wide range is used in the fields of industrial products or security.
If a sample 3 is provided so as to be distant from one beam source 1 that radially diffuses radiation in the width direction of the sample 3 as shown in FIG. 7A and radiation is irradiated in a wide range and detected by a radiation detector 2 such as a camera or a line sensor for covering a wide range, inspection in a wide range can be also performed by one beam source and one detector. In this case, even if air does not absorb radiation, the radiation dose is reduced substantially in proportion to the square of the distance. Accordingly, a beam source 1 having a high output is required.
The radiation tube current is increased, so that a large dose of radiation may be irradiated. Since a soft X-ray is absorbed by air, beam hardening occurs. The beam hardening is a phenomenon where when continuous radiation is transmitted through a material, low energy radiation is absorbed more than high energy radiation, so that an energy peak of the radiation is shifted to a high energy peak. It is not possible to inspect the sample at a distance due to the occurrence of beam hardening. When beam hardening occurs, the dose of absorbed radiation is not sufficient in the case of a sample having high transparency. Accordingly, detection sensitivity is not obtained. Further, a detection signal level is lowered, so that detection accuracy deteriorates. Further, in the soft X-ray inspection in the related art, a set of a beam source 1, a radiation detector 2, a sample 3, a conveying device, and the like is sealed in vacuum in a shield chamber. In the case of air, a sample 3 is installed close to the beam source 1 and a narrow range is observed.
FIG. 7B is a view showing another example of an inspection device in the related art that uses X-rays or β-rays. In FIG. 7B, a plurality of beam sources 1 is arranged in line so that the distance between the beam source 1 and the sample 3 is smaller than in the inspection device shown in FIG. 7A. Accordingly, a wide range is collectively detected.
If there is variation of the dose of radiation irradiated from the beam source 1, the measurement of the dose of transmitted radiation is directly affected. For this reason, measurement accuracy deteriorates. When it is to be determined whether an illegal device or an object, a metal piece, and the like exist in security management or food control, variation of several-percent in the dose of radiation irradiated from the beam source 1 is not a problem.
The stability of the dose of radiation irradiated from the beam source 1 is very important in the measurement of the thickness of a paper or resin sheet, a thin metal film, and the like. It may be possible to exclude the influence of the variation of the dose of radiation, which is irradiated from the beam source 1, by measuring the dose of radiation, which is not yet transmitted through the sample 3, and subtracting the dose of radiation, which is not yet transmitted through the sample 3, from the dose of radiation that has been transmitted through the sample 3. Accordingly, the transmittance of the sample 3 is accurately obtained.
In general, it may be possible to obtain transmission characteristics of the radiation in regard to a sample 3 using one detector, by measuring the dose of transmitted radiation in a state where a sample 3 exists and in a state where a sample 3 does not exist and taking a difference therebetween.
FIG. 7C is a view showing a measuring device of an in-line apparatus in the related art when the sample 3 is a product, such as a paper or resin sheet, continuously produced without being cut. Two radiation detectors 2a and 2b are installed so that a sample 3 is interposed therebetween. The detector 2a measures the dose of radiation that is not transmitted through a sample 3 yet. The detector 2a is disposed above the sample 3 on the side of the beam source 1. The detector 2b measures the dose of radiation transmitted through the sample 3. The detector 2b is disposed below the sample 3. Since the plurality of detectors 2a and 2b is provided as described above, it may be possible to measure and correct the variation of the dose of radiation in real time.
Another radiation detection apparatus is provided with one radiation detector that has a size larger than the width of the sample 3. In this case, the device measures online the dose of radiation, which is not yet transmitted through a sample 3, by emitting radiation to the range wider than the width of the sample 3 and measuring the dose of radiation at a portion where radiation directly reaches the radiation detector 2.
The beam source 1 has a limited lifetime, and needs to be replaced about once every several years. The beam source 1 is expensive. Since the beam source 1 is installed in a temperature control device, it is difficult to replace the beam source. For this reason, the beam source 1 requires large running costs. If a plurality of beam sources 1 is provided, running costs corresponding to the number of the beam sources are required. For this reason, the number of beam sources 1 needs to be reduced as much as possible.
Meanwhile, if a set of the beam source 1, the radiation detector 2, the sample 3, the conveying device, and the like is sealed in vacuum in the shield chamber as described above, the device needs to be strongly made as a whole. Accordingly, the price of the device becomes high as a whole. The input and output of the sample 3 are troublesome and time is required to form a vacuum. For this reason, the device is very poor in terms of use. Further, if the sample 3 is a continuous body such as a sheet, it is not easy to form a vacuum state.
In order to emit radiation in the wide range, the sample 3 needs to be distant from the beam source 1. Accordingly, since the amount of radiation absorbed by air is further increased and beam hardening occurs, it may not be possible to use radiation in the low-energy range that is advantageous for the measurement of transmission characteristics.
A gap between the beam source 1 and the sample 3 is generally open to the air. For this reason, the amount of radiation absorbed by air is changed due to the influence of the temperature of air, atmospheric pressure, humidity, and the like. This change becomes a factor of a measurement error.
Highly accurate measurement is particularly required for the measurement of the thickness of a thin film. Since a slight variation of the dose of radiation directly affects measurement accuracy, the control of the stability of the radiation irradiated from the beam source 1 is essential.
In general, it may be possible to cope with the change of the sensitivity of the detector and the short-term and long-term change of the dose of radiation by correcting the sensitivity of the detector or the variation of the dose of radiation, when the sample 3 is removed, every several hours to several days. The frequency of correction is high, the correction is troublesome, and the change of the dose of radiation cannot be fed back in real time. An expensive high-performance power supply for the beam source 1 is employed, and it necessary to manage the temperature of a measuring instrument or a beam source 1 and the temperature and humidity of an ambient environment.
If a plurality of radiation detectors is used as shown in FIG. 7C, larger costs, installation space, and the like are required.
Japanese Unexamined Patent Application, First Publication No. 2003-329430 discloses a line sensor or a radiation detector that can detect the variation of radiation irradiated from one or two beam sources 1 but cannot detect the radiation irradiated from three or more beam sources 1. Further, the line sensor measures the end portions of radiation flux. For example, the end portions of radiation flux are different from the middle portion of the radiation flux. Furthermore, the measured result is different from the variation of the total amount of the radiation flux.